Tachometer Calibration

None of the four tachs read the same on my Bayliner 4788. The Port pair was worst with over 200 RPM difference at 2300 RPM. I was going to purchase digital tachs, but I found a local business which, for a very reasonable price, recalibrated them for me. What impressed me was that he asked what RPM I nomally run at so that he could be sure they are all 100% exact at that rpm.
Shepard Instruments Langley BC
T 604 364 5845
Claudio Ugo

FYI I am not associated with this business.

Good news story, congratulations!

stanfromhell said:

None of the four tachs read the same on my Bayliner 4788. The Port pair was worst with over 200 RPM difference at 2300 RPM. I was going to purchase digital tachs, but I found a local business which, for a very reasonable price, recalibrated them for me. What impressed me was that he asked what RPM I nomally run at so that he could be sure they are all 100% exact at that rpm.
Shepard Instruments Langley BC
T 604 364 5845
Claudio Ugo

FYI I am not associated with this business.

Click to expand...

Be aware if you are not already that those old tachs will be accurate only at the RPM at which they were calibrated. They will be lesser and lesser so at speeds further and further away from that calibrated speed. On the other hand, once calibrated to your engine using a photo tach, a digital tach will be accurate throughout. I replaced my four old Stewart Warner's with digital tachs and was able to get them calibrated to read almost precisely with relative ease.

Recently, I changed a belt on one engine using a belt slightly smaller thus causing the alternator to spin slightly faster at a given RPM. Since I get the tach signal from the alternator, this caused that side to read 100 RPM faster. I noticed it immediately when I engaged the Glendinning synchronizer. I haven't had time to recalibrate but I know the actual from the other side so it is not a critical problem.

This is not a criticism of the choice you made. It's just a telling of another boater's choice and experience.

As a slow trawler (6.3 knot cruise).....

I only use 2 rpm settings anyhow.

Right at or within 100 of my cruise rpm of 1650.

Or zero.....for happy hour.

So fo me, calibration or accuracy isnt sll that big of a deal. The instructions said do it at my cruising rpm so I did and the rest are close enough.

On a twin the big deal is to have the tachs RPM close so a annoying harmonic is not created.

If all sounds smooth, feels smooth , you are close enough.


Should one be a purist , and can borrow a digital tach , a correction card like a compass deviation card could be created



Every 100 RPM should be accurate enough, and not take that long in the Hell Hole to do.

With synchronizer engaged my tachs disagree by 50 rpm throughout my cruising speeds. It never bothered me.

psneeld said:

As a slow trawler (6.3 knot cruise).....

I only use 2 rpm settings anyhow.

Right at or within 100 of my cruise rpm of 1650.

Or zero.....for happy hour.

So fo me, calibration or accuracy isnt sll that big of a deal. The instructions said do it at my cruising rpm so I did and the rest are close enough.

Click to expand...

Yup, fully understand but the difference in cost to repair vs new digital tachs may be inconsequential.

New , inexpensive tachs are a fraction of digital ones according to my research.

Accuracy of digitalis just wasnt important to me and if the OP had them calibrated for even less...hard to argue with.... in my book compared to sending thousands on critical systems cutting into the non critical budget columns.

But I know for some money is much less of an issue and I wish I could do some upgrades, this just wasnt one of them.

On a side note, I need to calibrate mines, they show a 200rpm diff between flybridge and lower helm. One reads from alt, one from a mechanical take of. Not sure which one has the correct reading but always base my cruising from flybridge one. I got an optical tach reader to Check correct form from engine pulley.
Question, to calibrate the gauge reading, is there a screw on the gauge back ( sorry if the question looks stupid but never did it).

L

psneeld said:

New , inexpensive tachs are a fraction of digital ones according to my research.

Accuracy of digitalis just wasnt important to me and if the OP had them calibrated for even less...hard to argue with.... in my book compared to sending thousands on critical systems cutting into the non critical budget columns.

But I know for some money is much less of an issue and I wish I could do some upgrades, this just wasnt one of them.

Click to expand...

Each person's definition of expensive is unique. A VDO 3,000 RPM programmable tach with hour meter can be had for $125.

Thats great, sorry I missed that when researching.

Blows a hole in my idea.

Every day we all come across the "new"......

Later that day....

Although after a quick review...the $150 say they are like the $65 ones in thet they connect to the alternator....and do not have thd 1% of less accuracy of true digital.

True digital (I think) like Aetnas, are almost $300 a pop for true 1rpm accuracy.

Thats $500 more for 4 tachs.

I am far more concerned with consistency than accuracy. My displacement speed is calculated as 9.475 kts. I run the boat up to 10 kts. Note the RPMs and then run long enough to get good fuel consumption numbers. If I don’t like the number I drop a 100 RPM’s and try again. Once I reach a speed consumption combination that i’m Happy with I have an RPM to run the boat at. That said, I do have Aetna Digital tachs.

psneeld said:

Thats great, sorry I missed that when researching.

Blows a hole in my idea.

Every day we all come across the "new"......

Later that day....

Although after a quick review...the $150 say they are like the $65 ones in thet they connect to the alternator....and do not have thd 1% of less accuracy of true digital.

True digital (I think) like Aetnas, are almost $300 a pop for true 1rpm accuracy.

Thats $500 more for 4 tachs.

Click to expand...

I have two different brands of programmable tachs, VDO and ISSPRO. Yes, the Aetnas are more precise, and quite expensive. However, I can tell you from personal experience, I was able to program those tachs with 15-20 RPM as measured with a photo tach. To my mind that is pretty close.

As for the source of the signal, I used my alternators through the the original wiring. Also, the programmable tachs can be using with any signal generator. If fact, using a signal generator with its known number of pulses makes the programming quite simple. So, sufficiently accurate modern tachs can be had for what I consider to be a reasonable price. And, once programmed they do not require periodic tweaking.

Lou_tribal said:

On a side note, I need to calibrate mines, they show a 200rpm diff between flybridge and lower helm. One reads from alt, one from a mechanical take of. Not sure which one has the correct reading but always base my cruising from flybridge one. I got an optical tach reader to Check correct form from engine pulley.
Question, to calibrate the gauge reading, is there a screw on the gauge back ( sorry if the question looks stupid but never did it).

L

Click to expand...

If your tach is a Motorola like mine then yes there is a little screw hiding behind a plastic button. Turn one way to increase reading the other way to decrease the reading.

My Tachs were US Marine and the screw on the back has 6 positions which are select the number of pulses per minute from the sender. These units must be sent away to be calibrated. I paid $70.00 CAD per tach which is a very reasonable price.

If you are not sure your tack is correct take a 10" piece of stiff piano wire to a friends boat with an accurate tack. Hold the wire on the engine, preferable with something metal - not your finger, while running at at say 1600 RPM. Slide the wire in and out and at a certain length the wire will vibrate the most. Mark that with a piece of tape, no tape on the vibrating part.


Now firmly hold the wire on your engine and when the wire vibrates the most your engine will be at 1600 RPM.